1. Field of the Invention
The present invention relates to an exhaust system that exhausts exhaust gas from an engine, and an engine device and a vehicle with the same.
2. Description of the Related Art
Conventionally, a catalyst device has been provided in an exhaust system to remove harmful substances contained in the exhaust gas emitted from an engine.
In order to quickly activate the catalyst device, the temperature of the catalyst needs to be rapidly increased in a short period of time. Consequently, there has been developed an exhaust system in which the catalyst device is arranged closer to the engine so that high temperature exhaust gas flows into the catalyst.
For example, in an exhaust system including a catalyst provided in a motorcycle described in JP 3242488 B, auxiliary oxidation catalysts are provided in a front exhaust pipe and a rear exhaust pipe connected to a front cylinder and a rear cylinder of a V-type two cylinder engine. However, in the configuration described in JP 3242488 B, the auxiliary oxidation catalysts need to be increased in number with an increase in the number of the cylinders of the engine, which increases the manufacturing cost.
As a method to solve such a problem, there is a method in which exhaust pipes of a plurality of cylinders are merged into one exhaust pipe and a catalyst device is provided at this portion where the exhaust pipes are merged. This can reduce the number of catalyst devices to be installed.
For example, in an exhaust treatment device of the exhaust gas of an internal combustion engine described in JP 2001-241323 A, exhaust pipes of first and third cylinders, in the order of ignition, are merged into one chamber to cause the exhaust gas to flow into a catalyst device through this chamber. Furthermore, exhaust pipes of second and fourth cylinders, in the order of ignition, are merged into the other chamber to cause the exhaust gas to flow into a catalyst device from this chamber.
Meanwhile, purification efficiency of the catalyst is significantly influenced by the air-fuel ratio of the engine. Therefore, in a conventional exhaust system, for example, oxygen sensors are arranged in the exhaust pipes so that components of the exhaust gas are detected. Based on the result of detection by the oxygen sensors, an optimization control is then applied to the air-fuel ratio of the engine, and a decrease in the purification efficiency of the catalyst is thus prevented.
However, in the exhaust system with a plurality of inflow portions of the exhaust gas to the catalyst device as described in JP 2001-241323 A, in order to detect the components of the exhaust gas exhausted from the respective cylinders of the engine with high accuracy, the oxygen sensors need to be provided in the inflow portions, respectively. For example, in the exhaust treatment device of the exhaust gas of the internal combustion engine described in JP 2001-241323 A, the oxygen sensors need to be provided in two chambers, respectively. In this case, the manufacturing cost is increased due to provision of a plurality of oxygen sensors.